clarify fractions
Final*Fake*: bugfix plotdir
FinalNewFake: major refactor

Refactor most of FinalNewFake (still to do get*Percentages impl).
Details:
- define local structs (SumWithErr2 and RegionLeptonFakeorreal) to
  simplify the input/output of the functions
- drop mu_percent_conv (just numberic noise; does not make physical sense)
- getFinalRate had two versions (1d and 2d); the 2d is probably a
  remnant from previous tests. Now everything is using the same
  implementation, and the 2d part is factorized to one (unused)
  function. All the histograms have nbinsy==1.
- getPercentages had two implementations. What they actually are doing
  is getFakePercentages and getRealPercentages. Now clarified.

At each step, verified that the output is consistent using the commands below:
FinalFakeHist \
 -t Oct_10 \
 -i out/fakerate/merged/ \
 -o /tmp/gerbaudo/FinalFakeHist_Oct_10.root \
 -p out/fakerate/merged/ \
 >& /tmp/gerbaudo/FinalFakeHist_Oct_10.log
echo "---diff log files---"
diff \
 /tmp/gerbaudo/FinalFakeHist_Oct_10.log \
 log/fakerate/FinalFakeHist_${TAG}.log
echo "---diff root files---"
./python/diff_ROOT_files.py \
 /tmp/gerbaudo/FinalFakeHist_Oct_10.root \
 out/fakerate/merged/FinalFakeHist_${TAG}.root